The field of the invention is magnetic resonance angiography (“MRA”), and particularly, studies of the human vasculature using contrast agents which enhance the NMR signals.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a “view” and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, or k-space samples, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 contrast enhanced (CE) MRA attempts to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CEMRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
As indicated above, the acquisition of MRA data is timed such that the central region of k-space is acquired as the bolus of contrast agent arrives in the arteries of interest. The ability to time the arrival of contrast varies considerably and it is helpful in many applications to acquire a series of MRA images in a dynamic study which depicts the separate enhancement of arteries and veins. A temporal series of images is also useful for observing delayed vessel filling patterns caused by disease. This requirement has been partially addressed by acquiring a series of time resolved images using a 3D “Fourier” acquisition as described by Korosec F., Frayne R, Grist T., Mistretta C., “Time-Resolved Contrast-Enhanced 3D MR Angiography”, Magn. Reson. Med. 1996; 36:345-351 and in U.S. Pat. No. 5,713,358. More recently, time-resolved MRA images have been acquired using a three-dimensional projection reconstruction method as described in U.S. Pat. No. 6,487,435 entitled “Magnetic Resonance Angiography Using Undersampled 3D Projection Imaging”.
With CEMRA image studies the usual practice is to acquire at least one image prior to the injection of contrast into the patient. This pre-contrast image is used as a mask that is subtracted from the contrast enhanced images to remove the signal from non-vascular tissues such as fat and bone. While this technique can be very effective, it does have two disadvantages. First, it extends the time of the scan and it requires that the patient be immobilized so that the mask image can be precisely registered with the contrast enhanced images from which it is subtracted. Any misregistration results in distracting image artifacts that may interfere with the diagnostic utility of the angiogram. The subtraction of two images also increases the standard deviation of the noise signal, reducing the signal-to-noise ratio (SNR) by the square root of 2.